1. Field of the Invention
The present invention relates generally to measurement of power distribution within a nuclear reactor core and, more particularly, is concerned with a system for measuring .sup.16 N decay gamma activity utilizing coolant flow from instrumentation tubes of the fuel assemblies which is routed through gamma detectors located outside of the nuclear reactor vessel.
2. Description of the Prior Art
In a typical pressurized water nuclear reactor (PWR), the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the top and bottom nozzles and a plurality of transverse grids axially spaced along the guide thimbles. Each fuel assembly is supported by its bottom nozzle on a lower core plate of the reactor core. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the grids between the top and bottom nozzles.
The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Further, in each fuel assembly, provisions are made to enable actual flux distribution measurements to be taken under reactor operating conditions to determine the power distribution of each fuel assembly and thus the overall reactor. For this purpose a hollow instrumentation tube is located in each fuel assembly, extending between the bottom and top nozzles. The instrumentation tube is open at its bottom end and aligned with and seated within a passage extending through the adapter plate of the bottom nozzle.
Elongated flux thimble tubes housing flux detectors are inserted into the reactor core through conduits extending from the bottom of the reactor vessel. Each thimble tube is extended upwardly from the lower core plate to the bottom nozzle of each fuel assembly to where it is inserted into the instrumentation tube through the open bottom end thereof. The thimble tube is adapted to take the flux distribution measurements within the fuel assembly.
During operation, the thimble tubes are stationary. They are retracted from the reactor core only under depressurized conditions during refueling or for maintenance. Primary reactor coolant flows upwardly through the annulus formed between the outside diameter of each flux thimble tube and the inside diameter of the instrumentation tube of each fuel assembly, entering the annulus from the underside of the bottom nozzle.
The environment within the instrumentation tubes within the nuclear reactor core is a severe one. The detectors deployed in the thimble tubes and subjected to this environment are adversely affected such that their useful life is shortened. Consequently, a need exists for an alternative approach to measuring power distribution without placing the detectors within the reactor core environment.